package syntax
import "regexp/syntax"
Package syntax parses regular expressions into parse trees and compiles parse trees into programs. Most clients of regular expressions will use the facilities of package regexp (such as Compile and Match) instead of this package.
Syntax
The regular expression syntax understood by this package when parsing with the Perl flag is as follows. Parts of the syntax can be disabled by passing alternate flags to Parse.
Single characters:
. any character, possibly including newline (flag s=true)
[xyz] character class
[^xyz] negated character class
\d Perl character class
\D negated Perl character class
[[:alpha:]] ASCII character class
[[:^alpha:]] negated ASCII character class
\pN Unicode character class (one-letter name)
\p{Greek} Unicode character class
\PN negated Unicode character class (one-letter name)
\P{Greek} negated Unicode character class
Composites:
xy x followed by y x|y x or y (prefer x)
Repetitions:
x* zero or more x, prefer more
x+ one or more x, prefer more
x? zero or one x, prefer one
x{n,m} n or n+1 or ... or m x, prefer more
x{n,} n or more x, prefer more
x{n} exactly n x
x*? zero or more x, prefer fewer
x+? one or more x, prefer fewer
x?? zero or one x, prefer zero
x{n,m}? n or n+1 or ... or m x, prefer fewer
x{n,}? n or more x, prefer fewer
x{n}? exactly n x
Implementation restriction: The counting forms x{n,m}, x{n,}, and x{n} reject forms that create a minimum or maximum repetition count above 1000. Unlimited repetitions are not subject to this restriction.
Grouping:
(re) numbered capturing group (submatch) (?P<name>re) named & numbered capturing group (submatch) (?<name>re) named & numbered capturing group (submatch) (?:re) non-capturing group (?flags) set flags within current group; non-capturing (?flags:re) set flags during re; non-capturing Flag syntax is xyz (set) or -xyz (clear) or xy-z (set xy, clear z). The flags are: i case-insensitive (default false) m multi-line mode: ^ and $ match begin/end line in addition to begin/end text (default false) s let . match \n (default false) U ungreedy: swap meaning of x* and x*?, x+ and x+?, etc (default false)
Empty strings:
^ at beginning of text or line (flag m=true) $ at end of text (like \z not \Z) or line (flag m=true) \A at beginning of text \b at ASCII word boundary (\w on one side and \W, \A, or \z on the other) \B not at ASCII word boundary \z at end of text
Escape sequences:
\a bell (== \007)
\f form feed (== \014)
\t horizontal tab (== \011)
\n newline (== \012)
\r carriage return (== \015)
\v vertical tab character (== \013)
\* literal *, for any punctuation character *
\123 octal character code (up to three digits)
\x7F hex character code (exactly two digits)
\x{10FFFF} hex character code
\Q...\E literal text ... even if ... has punctuation
Character class elements:
x single character
A-Z character range (inclusive)
\d Perl character class
[:foo:] ASCII character class foo
\p{Foo} Unicode character class Foo
\pF Unicode character class F (one-letter name)
Named character classes as character class elements:
[\d] digits (== \d)
[^\d] not digits (== \D)
[\D] not digits (== \D)
[^\D] not not digits (== \d)
[[:name:]] named ASCII class inside character class (== [:name:])
[^[:name:]] named ASCII class inside negated character class (== [:^name:])
[\p{Name}] named Unicode property inside character class (== \p{Name})
[^\p{Name}] named Unicode property inside negated character class (== \P{Name})
Perl character classes (all ASCII-only):
\d digits (== [0-9]) \D not digits (== [^0-9]) \s whitespace (== [\t\n\f\r ]) \S not whitespace (== [^\t\n\f\r ]) \w word characters (== [0-9A-Za-z_]) \W not word characters (== [^0-9A-Za-z_])
ASCII character classes:
[[:alnum:]] alphanumeric (== [0-9A-Za-z])
[[:alpha:]] alphabetic (== [A-Za-z])
[[:ascii:]] ASCII (== [\x00-\x7F])
[[:blank:]] blank (== [\t ])
[[:cntrl:]] control (== [\x00-\x1F\x7F])
[[:digit:]] digits (== [0-9])
[[:graph:]] graphical (== [!-~] == [A-Za-z0-9!"#$%&'()*+,\-./:;<=>?@[\\\]^_`{|}~])
[[:lower:]] lower case (== [a-z])
[[:print:]] printable (== [ -~] == [ [:graph:]])
[[:punct:]] punctuation (== [!-/:-@[-`{-~])
[[:space:]] whitespace (== [\t\n\v\f\r ])
[[:upper:]] upper case (== [A-Z])
[[:word:]] word characters (== [0-9A-Za-z_])
[[:xdigit:]] hex digit (== [0-9A-Fa-f])
Unicode character classes are those in unicode.Categories and unicode.Scripts.
Index
- func IsWordChar(r rune) bool
- type EmptyOp
- type Error
- type ErrorCode
- type Flags
- type Inst
- type InstOp
- type Op
- type Prog
- type Regexp
Functions
func IsWordChar
func IsWordChar(r rune) bool
IsWordChar reports whether r is considered a “word character” during the evaluation of the \b and \B zero-width assertions. These assertions are ASCII-only: the word characters are [A-Za-z0-9_].
Types
type EmptyOp
type EmptyOp uint8
An EmptyOp specifies a kind or mixture of zero-width assertions.
const ( EmptyBeginLine EmptyOp = 1 << iota EmptyEndLine EmptyBeginText EmptyEndText EmptyWordBoundary EmptyNoWordBoundary )
func EmptyOpContext
func EmptyOpContext(r1, r2 rune) EmptyOp
EmptyOpContext returns the zero-width assertions satisfied at the position between the runes r1 and r2. Passing r1 == -1 indicates that the position is at the beginning of the text. Passing r2 == -1 indicates that the position is at the end of the text.
type Error
type Error struct { Code ErrorCode Expr string }
An Error describes a failure to parse a regular expression and gives the offending expression.
func (*Error) Error
func (e *Error) Error() string
type ErrorCode
type ErrorCode string
An ErrorCode describes a failure to parse a regular expression.
const ( // Unexpected error ErrInternalError ErrorCode = "regexp/syntax: internal error" // Parse errors ErrInvalidCharClass ErrorCode = "invalid character class" ErrInvalidCharRange ErrorCode = "invalid character class range" ErrInvalidEscape ErrorCode = "invalid escape sequence" ErrInvalidNamedCapture ErrorCode = "invalid named capture" ErrInvalidPerlOp ErrorCode = "invalid or unsupported Perl syntax" ErrInvalidRepeatOp ErrorCode = "invalid nested repetition operator" ErrInvalidRepeatSize ErrorCode = "invalid repeat count" ErrInvalidUTF8 ErrorCode = "invalid UTF-8" ErrMissingBracket ErrorCode = "missing closing ]" ErrMissingParen ErrorCode = "missing closing )" ErrMissingRepeatArgument ErrorCode = "missing argument to repetition operator" ErrTrailingBackslash ErrorCode = "trailing backslash at end of expression" ErrUnexpectedParen ErrorCode = "unexpected )" ErrNestingDepth ErrorCode = "expression nests too deeply" ErrLarge ErrorCode = "expression too large" )
func (ErrorCode) String
func (e ErrorCode) String() string
type Flags
type Flags uint16
Flags control the behavior of the parser and record information about regexp context.
const ( FoldCase Flags = 1 << iota // case-insensitive match Literal // treat pattern as literal string ClassNL // allow character classes like [^a-z] and [[:space:]] to match newline DotNL // allow . to match newline OneLine // treat ^ and $ as only matching at beginning and end of text NonGreedy // make repetition operators default to non-greedy PerlX // allow Perl extensions UnicodeGroups // allow \p{Han}, \P{Han} for Unicode group and negation WasDollar // regexp OpEndText was $, not \z Simple // regexp contains no counted repetition MatchNL = ClassNL | DotNL Perl = ClassNL | OneLine | PerlX | UnicodeGroups // as close to Perl as possible POSIX Flags = 0 // POSIX syntax )
type Inst
type Inst struct { Op InstOp Out uint32 // all but InstMatch, InstFail Arg uint32 // InstAlt, InstAltMatch, InstCapture, InstEmptyWidth Rune []rune }
An Inst is a single instruction in a regular expression program.
func (*Inst) MatchEmptyWidth
func (i *Inst) MatchEmptyWidth(before rune, after rune) bool
MatchEmptyWidth reports whether the instruction matches an empty string between the runes before and after. It should only be called when i.Op == InstEmptyWidth.
func (*Inst) MatchRune
func (i *Inst) MatchRune(r rune) bool
MatchRune reports whether the instruction matches (and consumes) r. It should only be called when i.Op == InstRune.
func (*Inst) MatchRunePos
func (i *Inst) MatchRunePos(r rune) int
MatchRunePos checks whether the instruction matches (and consumes) r. If so, MatchRunePos returns the index of the matching rune pair (or, when len(i.Rune) == 1, rune singleton). If not, MatchRunePos returns -1. MatchRunePos should only be called when i.Op == InstRune.
func (*Inst) String
func (i *Inst) String() string
type InstOp
type InstOp uint8
An InstOp is an instruction opcode.
const ( InstAlt InstOp = iota InstAltMatch InstCapture InstEmptyWidth InstMatch InstFail InstNop InstRune InstRune1 InstRuneAny InstRuneAnyNotNL )
func (InstOp) String
func (i InstOp) String() string
type Op
type Op uint8
An Op is a single regular expression operator.
const ( OpNoMatch Op = 1 + iota // matches no strings OpEmptyMatch // matches empty string OpLiteral // matches Runes sequence OpCharClass // matches Runes interpreted as range pair list OpAnyCharNotNL // matches any character except newline OpAnyChar // matches any character OpBeginLine // matches empty string at beginning of line OpEndLine // matches empty string at end of line OpBeginText // matches empty string at beginning of text OpEndText // matches empty string at end of text OpWordBoundary // matches word boundary `\b` OpNoWordBoundary // matches word non-boundary `\B` OpCapture // capturing subexpression with index Cap, optional name Name OpStar // matches Sub[0] zero or more times OpPlus // matches Sub[0] one or more times OpQuest // matches Sub[0] zero or one times OpRepeat // matches Sub[0] at least Min times, at most Max (Max == -1 is no limit) OpConcat // matches concatenation of Subs OpAlternate // matches alternation of Subs )
func (Op) String
func (i Op) String() string
type Prog
type Prog struct { Inst []Inst Start int // index of start instruction NumCap int // number of InstCapture insts in re }
A Prog is a compiled regular expression program.
func Compile
func Compile(re *Regexp) (*Prog, error)
Compile compiles the regexp into a program to be executed. The regexp should have been simplified already (returned from re.Simplify).
func (*Prog) Prefix
func (p *Prog) Prefix() (prefix string, complete bool)
Prefix returns a literal string that all matches for the regexp must start with. Complete is true if the prefix is the entire match.
func (*Prog) StartCond
func (p *Prog) StartCond() EmptyOp
StartCond returns the leading empty-width conditions that must be true in any match. It returns ^EmptyOp(0) if no matches are possible.
func (*Prog) String
func (p *Prog) String() string
type Regexp
type Regexp struct { Op Op // operator Flags Flags Sub []*Regexp // subexpressions, if any Sub0 [1]*Regexp // storage for short Sub Rune []rune // matched runes, for OpLiteral, OpCharClass Rune0 [2]rune // storage for short Rune Min, Max int // min, max for OpRepeat Cap int // capturing index, for OpCapture Name string // capturing name, for OpCapture }
A Regexp is a node in a regular expression syntax tree.
func Parse
func Parse(s string, flags Flags) (*Regexp, error)
Parse parses a regular expression string s, controlled by the specified Flags, and returns a regular expression parse tree. The syntax is described in the top-level comment.
func (*Regexp) CapNames
func (re *Regexp) CapNames() []string
CapNames walks the regexp to find the names of capturing groups.
func (*Regexp) Equal
func (x *Regexp) Equal(y *Regexp) bool
Equal reports whether x and y have identical structure.
func (*Regexp) MaxCap
func (re *Regexp) MaxCap() int
MaxCap walks the regexp to find the maximum capture index.
func (*Regexp) Simplify
func (re *Regexp) Simplify() *Regexp
Simplify returns a regexp equivalent to re but without counted repetitions and with various other simplifications, such as rewriting /(?:a+)+/ to /a+/. The resulting regexp will execute correctly but its string representation will not produce the same parse tree, because capturing parentheses may have been duplicated or removed. For example, the simplified form for /(x){1,2}/ is /(x)(x)?/ but both parentheses capture as $1. The returned regexp may share structure with or be the original.
func (*Regexp) String
func (re *Regexp) String() string